testing: make Eq(nan,<meh>) return False, Ne(nan, <meh>) return True

Not necessarily intended for a merge...
`Eq` is not same as `==`   (mathematical vs structural)
`Ne` is not same as `!=`    (mathematical vs structural)

But @asmeurer says `Lt` should be same `<`

nan == nan is true (by sympy structural equality), but IEEE floating point says nan not mathematically equal to nan.  This is my PR: essentially for compatibility with IEEE floating point.

This PR is unrelated to my other one about `Lt` and `<`.

I'm not a fan of random tests when all cases can be enumerated but if you want...